Pseudowire (PW) switching type-length-value (TLV)

ABSTRACT

A method and apparatus for providing information in a network by way of a Pseudowire switching TLV is presented. A second device in the network receives a first message having information relating to a first device in the network. The second device appends information relating to the second device (by way of a Pseudowire switching TLV) to the first message, resulting in a second message. The second message is then forwarded to a third device in the network.

BACKGROUND

Computer networks have become ubiquitous. Computer networks include theInternet, Service Provider (SP) networks, private networks, and LocalArea Networks (LANs). A network such as an SP network may includeperipherally located Provider Edge (PE) routers, each of which couplesto one or multiple Customer Edge (CE) routers. For the core network, aningress PE uses BGP functions to determine the egress PE.

Virtual Private Networks (VPNs) provide a secured means for transmittingand receiving data between network nodes even though a correspondingphysical network supporting propagation of the data is shared by manyusers (and VPNs). In a typical networking environment used for routingdata, the environment may include a number of Customer Edge (CE)routers, a number of Provider Edge (PE) routers and a packet-switchednetwork (PSN). Data, encapsulated in layer-2 frames, may be forwardedfrom a first CE router to a first PE router, from the first PE routeracross the PSN to a second PE router, and from the second PE router to asecond CE router. A Pseudowire (PW) may be utilized to transfer dataacross the PSN. A Pseudowire is a mechanism that emulates attributes ofa service such as Asynchronous Transfer Mode (ATM), Frame Relay (FR),Point-to-Point Protocol (PPP), High Level Data Link Control (HDLC),Synchronous Optical Network (SONET) Frames or Ethernet over a PSN. Thefunctions provided by the PW include encapsulating Protocol Data Units(PDUs) arriving at an ingress port, carrying them across a path ortunnel, managing their timing and order, and any other operationsrequired to emulate the behavior and characteristics of the particularservice. In a particular embodiment, PWs are used to carry ingresslayer-2 traffic from an ingress PE router to an egress PE router, andthen forward the layer-2 traffic out of an egress port of the egress PErouter.

The signaling and encapsulation techniques for establishing SingleSegment Pseudowires (SS-PWs) between a pair of Ultimate PEs (U-PEs) iswell known. In some cases an SS-PW is insufficient. In such a case, aMulti-Segment Pseudowire (MS-PW) composed of more than one SS-PW,traversing one or more Switching Point PEs (SP-PEs) may be used.

SUMMARY

Conventional mechanisms such as those explained above suffer from avariety of deficiencies. One such deficiency is that in conventionalnetwork systems using MS-PWs, an MS-PW may traverse several switchingpoints, involving multiple routers in separate administrative domains.The PWs may eventually lose context of where the MS-PW originated, howmany hops it has traversed, etc. This makes troubleshooting andmanagement difficult, particularly across administrative domains whereaccess to all equipment participating in the MS-PW's establishment maybe limited.

In some environments it may not be possible, desirable or feasible toestablish a PW control channel between the ultimate source anddestination PEs. At a minimum PW control channel establishment requiresknowledge of and reachability to the remote (ultimate) PE IP address.The local (ultimate) PE may not have access to this information relatedto topology, operational or security constraints.

Another deficiency involves a single Autonomous system (AS) where the PWsetup path between the ultimate PEs is computed by an external entity(i.e., a client-layer routing protocol). For example, given a full meshof PWE3 control channels established between PE-A, PE-B and PE-C. Aclient-layer L2 connection tunneled through a PW is required betweenultimate PE-A and PE-C. The external entity computes a PW setup paththat passes through PE-B. This results in two discrete PW segments beingbuilt: one between PE-A and PE-B and one between PE-B and PE-C. Thesuccessful client-layer L2 connection between ultimate PE-A and ultimatePE-C requires that PE-B performs the PWE3 switching process.

Embodiments of the invention significantly overcome such deficienciesand provide mechanisms and techniques that provide a Pseudowire (PW)switching Type-Length-Value (TLV) which is used to provide informationacross a network which includes information regarding the differentrouters in the path that make up a MS-PW. The PW switching TLV can beused to facilitate troubleshooting, operation and management of MS-PWsby providing information at each PE about the originating and switchingpoint PEs along an MS-PW path. The PW switching TLV can further be usedfor passing PW status messages pertaining to a specific U-PE or SP-PEalong an MS-PW. The ability to document the actual path that a PW setupmessage utilizes is very important for managing a complex network ofPWs, particularly when spanning different providers. By way of thepresent invention, a PE can receive specific status messages from anintermediate S-PE or U-PE nodes, and take appropriate action. The PWswitching TLV transmits more then just a route as the description, thePW ID of the SS-PW traversed, as well as identification information foreach SP-PE are also transmitted. The PW switching TLV can be used inconjunction with PW status messages to indicate which specific nodealong the path has sent the PW status message.

As described above, a MS-PW is composed of more than one SS-PW,traversing one or more Switching Point PEs (SP-PEs). When traversingeach SP-PEs, information from the U-PEs and SP-PEs may be gathered andpassed along to the next PE. This information aids greatly introubleshooting and provisioning of MS-PWs.

The edge-to-edge PW might traverse several switching points, in separateadministrative domains. For management and troubleshooting reasons it isuseful to record all the switching points that the PW traverses. This isaccomplished by using a PW switching point TLV.

In a particular embodiment of a method of providing information in anetwork, the method includes receiving, at a second device in thenetwork, a first message having information relating to a first devicein the network. The method includes appending, by the second device,information relating to the second device to the first message,resulting in a second message. The method further includes forwardingthe second message to a third device in the network.

Other embodiments include a method of determining a path between networkdevices. This method includes receiving, at an intermediate networkdevice located between a source network device and a destination networkdevice, a first message including upstream path information between theintermediate device and the source device. The method further includesreceiving at the intermediate network device a second message includingdownstream path information between the intermediate device and thedestination device. The method additionally includes combining, at theintermediate network device, the upstream path information and thedownstream path information to determine a complete path from the sourcenetwork device to the destination network device which includes theintermediate device.

Still other embodiments include a computerized device or system,configured to process all the method operations disclosed herein asembodiments of the invention. In such embodiments, the computerizeddevice or system includes a memory system, a processor, communicationsinterface in an interconnection mechanism connecting these components.The memory system is encoded with a process that provides and utilizes aPW switching TLV as explained herein that when performed (e.g. whenexecuting) on the processor, operates as explained herein within thecomputerized device to perform all of the method embodiments andoperations explained herein as embodiments of the invention. Thus anycomputerized device or system that performs or is programmed to performup processing explained herein is an embodiment of the invention.

Other arrangements of embodiments of the invention that are disclosedherein include software programs to perform the method embodiment stepsand operations summarized above and disclosed in detail below. Moreparticularly, a computer program product is one embodiment that has acomputer-readable medium including computer program logic encodedthereon that when performed in a computerized device provides associatedoperations providing a Pseudowire switching TLV as explained herein. Thecomputer program logic, when executed on at least one processor with acomputing system, causes the processor to perform the operations (e.g.,the methods) indicated herein as embodiments of the invention. Sucharrangements of the invention are typically provided as software, codeand/or other data structures arranged or encoded on a computer readablemedium such as an optical medium (e.g., CD-ROM), floppy or hard disk orother a medium such as firmware or microcode in one or more ROM or RAMor PROM chips or as an Application Specific Integrated Circuit (ASIC) oras downloadable software images in one or more modules, sharedlibraries, etc. The software or firmware or other such configurationscan be installed onto a computerized device to cause one or moreprocessors in the computerized device to perform the techniquesexplained herein as embodiments of the invention. Software processesthat operate in a collection of computerized devices, such as in a groupof data communications devices or other entities can also provide thesystem of the invention. The system of the invention can be distributedbetween many software processes on several data communications devices,or all processes could run on a small set of dedicated computers, or onone computer alone.

It is to be understood that the embodiments of the invention can beembodied strictly as a software program, as software and hardware, or ashardware and/or circuitry alone, such as within a data communicationsdevice. The features of the invention, as explained herein, may beemployed in data communications devices and/or software systems for suchdevices such as those manufactured by Cisco Systems, Inc. of San Jose,Calif.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates an example network topology showing a network havinga multi-segment Pseudowire extending from a first ultimate PE device toanother ultimate PE device in accordance with embodiments of the presentinvention;

FIG. 2 illustrates a flowchart for a particular embodiment of a methodof providing and utilizing a Pseudowire switching TLV;

FIG. 3 illustrates a flowchart for a particular embodiment of a methodof determining a path between network devices; and

FIG. 4 illustrates an example network device architecture that providesand utilizes a Pseudowire switching TLV in accordance with embodimentsof the invention.

DETAILED DESCRIPTION

In order to utilize a Pseudowire (PW), the PW must first be established.In order to establish a PW, a Virtual Private Wire Service (VPWS) PE isconfigured with the Target Attachment identifier of the remote PE andthe IP address of the remote PE. The PE then performs a lookup in thelocal Multi Packet Label Switching Label Forwarding Information Base(MPLS LFIB) for a Next Hop Label Forwarding Entry (NHLFE) that containsa Label Switching Path (LSP) to reach the destination PE. If an LSP isfound, the PW is set up.

A “PW switching point” is a mechanism (in the form of a TLV) whichcarries information about each PE over the MS-PW signalinginfrastructure. During a Multi-Segment Pseudowire (MS-PW) establishment,information about the originating ultimate PE (U-PE) and each switchingpoint provider edge router (SP-PE) is gathered into a TLV, appended andpassed to the next PE. Each PE then copies information from the previousPE, appending its own information for the next PE and so on. Thedestination U-PE then has a full record of the various SP-PEs that thePW has traversed, as well as the originating U-PE.

The SP-TLV is not only used during PW establishment, but can also beused for identifying the origin of a status notification message aswell. Status notification messages are used to indicate state changes toan interface or PW without actually tearing down the PW (for example,signaling that an interface is temporarily disabled, an alarm, etc.).This same SP-TLV is included with the Status Message to indicate theorigin of the message. This allows any PE (whether a U-PE or an SP-PE)to send a status message, and for the destination PE to know where themessage originated. SP-TLVs may be appended at each PE as during setup,or simply passed without appending new TLVs at each point (in the lattercase, that this information has not changed since PW setup and should becached at the U-PE).

The actual format of the SP-TLV could take on many forms, and thefollowing is one example, provided for purposes of explanation.

The PW switching point TLV is appended to the PW FEC at each switchingpoint.

The PW sw TLV Length field specifies the total length of all thefollowing PW switching point TLV fields in octets. The Type fieldencodes how the Value field is to be interpreted. The Length fieldspecifies the length of the Value field in octets.

At the PW switching point there is no attachment circuit, but insteadtwo PWs connected together. Similarly, the status of the PWs isforwarded unchanged from one PW to the other by the control planeswitching function. However, it may sometimes be necessary tocommunicate the status of one of the locally attached Single HopPseudowire (SH-PW) at a PW switching point.

The Type Values are assigned as follows: Type Length Description 0x00 0Reserved 0x01 4 PW ID of last PW traversed 0x02 variable PW SwitchingPoint description string 0x03 4 IP address of PW Switching Point(Optional)

For LDP this can be accomplished by sending an LDP status notificationmessage containing the PW status TLV, as well as a PW switching pointTLV, an example of which is shown as follows.

It should be noted that the SP-TLV is appended to the end of the statusmessage, as shown above.

Referring now to FIG. 1, a network 10 implementing a particularembodiment of the present method for using Pseudowire switching TLVs ina MS-PW is shown. In this network 10, a first Ultimate provider edgerouter (U-PE1), also referred to as an originating U-PE, is shown incommunication with a first service provider network (service providernetwork1). Also in communication with U-PE1 is SP-PE1 which communicateswith U-PE1 by way of service provider network1. SP-PE1 is also incommunication with SP-PE2, which is part of a second service providernetwork (service provider network 2). SP-PE2 is further in communicationwith U-PE2, also referred to as a destination U-PE, via service providernetwork2. It should be appreciated that while only a three segment MS-PW(comprised of segments 14, 16, and 18) is shown and described, thepresent invention should not be limited to a three segment MS-PW and thedescription of the present invention applies to an MS-PW have any numberof segments and any number of SP-PEs.

In operation, a MS-PW is set up including multiple segments 14, 16 and18. Part of the messaging used to setup the MS-PW involves a setupmessage. The message is originated at U-PE1 and includes a first TLVwhich includes information regarding U-PE1. This information may includea PW ID for identifying the particular MS-PW as well as other relatedinformation. This first TLV is received by the next hop, in thisinstance SP-PE1. SP-PE1 appends a second TLV to the first TLV to providea new message. The second TLV contains information relating to SP-PE1.The information relating to SP-PE1 may include routing information, adescription of the route and identification information of the PE.

This new message is then forwarded to the next hop. The next hop in thisexample is SP-PE2, which takes the message having the first TLV and thesecond TLV and appends a third TLV to it to provide a new message. Thethird TLV contains information relating to SP-PE2. The informationrelating to SP-PE2 may include routing information, a description of theroute and identification information of the PE.

This new message is then forwarded to the next hop. The next hop in thisinstance is U-PE2. U-PE2 now has information relating to the originationof the MS-PW and all the hops between the originating U-PE and thedestination U-PE. While the path beginning at U-PE1 and ending at U-PE2has been described, the present invention is bi-directional and alsoworks when a message is initiated at U-PE2 and traverses the MS-PW toU-PE1.

A similar procedure takes place when status messages are transmittedacross the MS-PW. A status message can be originated by any PE of thepath, and is used to indicate to other PEs the occurrence of a link ornext hop failure. For a status message, an SP-PE may append its TLV toan originator's TLV or may pass the originators TLV directly throughwithout appending its TLV thereto (for example, if the information hasnot changed since the MS-PW was setup and the information is thereforecached at the U-PE).

Referring again to FIG. 1, if SP-PE1 where to originate a statusmessage, SP-PE1 would send the status message downstream to SP-PE2.SP-PE2 could either append its TLV to the message and send the messageincluding the appended TLV to U-PE2 or could determine not to append itsTLV to the status message and pass the message straight thru to U-PE2.

Since the chain of paths is bi-directional between the U-PEs, theinformation relating to the total path from end to end is available ateach node of the path. For example, SP-PE2, since it has receivedmessages forwarded downstream for the MS-PW and has also receivedmessages upstream for the MS-PW, may take the downstream chain of nodesand combine it with the upstream chain of nodes (e.g. by using the PWID)and produce a chain of nodes which described the MS-PW from end to end.

A flow chart of the presently disclosed methods is depicted in FIGS. 2and 3. The rectangular elements are herein denoted “processing blocks”and represent computer software instructions or groups of instructions.Alternatively, the processing blocks represent steps performed byfunctionally equivalent circuits such as a digital signal processorcircuit or an application specific integrated circuit (ASIC). The flowdiagrams do not depict the syntax of any particular programminglanguage. Rather, the flow diagrams illustrate the functionalinformation one of ordinary skill in the art requires to fabricatecircuits or to generate computer software to perform the processingrequired in accordance with the present invention. It should be notedthat many routine program elements, such as initialization of loops andvariables and the use of temporary variables are not shown. It will beappreciated by those of ordinary skill in the art that unless otherwiseindicated herein, the particular sequence of steps described isillustrative only and can be varied without departing from the spirit ofthe invention. Thus, unless otherwise stated the steps described beloware unordered meaning that, when possible, the steps can be performed inany convenient or desirable order.

Referring now to FIG. 2, a method 100 of providing information in anetwork is shown. The method 100 begins, as shown in processing block102, with receiving, at a second device in the network, a first messagehaving information relating to a first device in the network. The firstdevice may be a U-PE and the second device may be a SP-PE.

As shown in processing block 104, receiving a first message comprisesreceiving a first message including at least one of routing information,a description of the route, and identification information of the firstdevice. In processing block 106, the receiving a first message includesreceiving a first message comprising one of a status message and a setupmessage. A setup message is used to establish the MS-PW, and a statusmessage is sent by any node to peer nodes to indicate a link or next hopfailure. As shown in processing block 108, the first message may includeinformation relating to other devices upstream of the second device. Forexample, the message may include a TLV for the U-PE and TLVs for anyupstream nodes in the path between the U-PE and the second device.

In processing block 110 the second device appends information relatingto the second device to the first message, resulting in a secondmessage. As recited in processing block 112, the appended informationcan be realized as a TLV which includes at least one of routinginformation, a description of the route and identification informationof the second device.

In processing block 114 the second message is forwarded to a thirddevice in the network. The second message comprises the first messageand the TLV of the second device. The third device may be a U-PE oranother SP-PE. In processing block 116, the first device, the seconddevice and the third device are part of a Multi-Segment Pseudowire(MS-PW).

By way of the above method, the actual path that a PW setup messageutilizes is documented and can be used for managing a complex network ofPWs, particularly when spanning different providers. Further, by way ofthe above method, a PE can receive specific status messages from anintermediate S-PE or U-PE nodes, and take appropriate action. The PWswitching TLV can be used in conjunction with PW status messages toindicate which specific node along the path has sent the PW statusmessage.

Referring now to FIG. 3, a method 150 of determining a path betweennetwork devices is shown. The method begins with processing block 152wherein a first message is received at an intermediate network devicelocated between a source network device and a destination networkdevice, the first message including upstream path information betweenthe intermediate device and the source device. The source network devicemay be a U-PE and the intermediate network device may be a SP-PE.

As shown in processing block 154, the first message includes at leastone of routing information, a description of the route andidentification information of the source device. Further, as shown inprocessing block 156 the first message may include information relatingto other upstream devices between the source device and the intermediatenetwork device. This may include one or more TLVs.

In processing block 158 the intermediate network device receives asecond message including downstream path information between theintermediate device and the destination device. As recited in processingblock 160, the second message includes at least one of routinginformation, a description of the route and identification informationof the destination device. In processing block 162, the second messagemay include information relating to other downstream devices between thedestination device and the intermediate network device. This may includeone or more TLVs.

In processing block 164, the downstream path information and theupstream path information are combined at the intermediate networkdevice to determine a complete path from the source network device tothe destination network device which includes the intermediate device.As shown in processing block 166 the source device, the intermediatedevice and the destination device are part of a Multi-Segment Pseudowire(MS-PW).

By way of the above-described method, every node in a path of nodes thatmake up a MS-PW can determine the entire path of nodes that make up theMS-PW.

FIG. 4 illustrates example architectures of a network device 240. Inthis example, the device 240 includes an interconnection mechanism 211that couples a memory system 212, a processor 213, and a communicationsinterface 214. The communications interface 214 allows the device 240 tocommunicate with external devices or systems.

The memory system 212 may be any type of computer readable medium thatis encoded with an application 255-A that represents software code suchas data and/or logic instructions (e.g., stored in the memory or onanother computer readable medium such as a disk) that embody theprocessing functionality of embodiments of the invention for the agent255 as explained above. The processor 213 can access the memory system212 via the interconnection mechanism 211 in order to launch, run,execute, interpret or otherwise perform the logic instructions of theapplications 255-A for the host in order to produce a correspondingprocess 255-B. In other words, the process 255-B represents one or moreportions of the application 255-A performing within or upon theprocessor 213 in the device 240.

It is to be understood that embodiments of the invention include theapplications (i.e., the un-executed or non-performing logic instructionsand/or data) encoded within a computer readable medium such as a floppydisk, hard disk or in an optical medium, or in a memory type system suchas in firmware, read only memory (ROM), or, as in this example, asexecutable code within the memory system 212 (e.g., within random accessmemory or RAM). It is also to be understood that other embodiments ofthe invention can provide the applications operating within theprocessor 213 as the processes. While not shown in this example, thoseskilled in the art will understand that the computer system may includeother processes and/or software and hardware components, such as anoperating system, which have been left out of this illustration for easeof description of the invention.

Having described preferred embodiments of the invention it will nowbecome apparent to those of ordinary skill in the art that otherembodiments incorporating these concepts may be used. Additionally, thesoftware included as part of the invention may be embodied in a computerprogram product that includes a computer useable medium. For example,such a computer usable medium can include a readable memory device, suchas a hard drive device, a CD-ROM, a DVD-ROM, or a computer diskette,having computer readable program code segments stored thereon. Thecomputer readable medium can also include a communications link, eitheroptical, wired, or wireless, having program code segments carriedthereon as digital or analog signals. Accordingly, it is submitted thatthat the invention should not be limited to the described embodimentsbut rather should be limited only by the spirit and scope of theappended claims.

1. A method of providing information in a network comprising: receiving,at a second device in the network, a first message having informationrelating to a first device in the network; appending, by the seconddevice, information relating to the second device to the first message,resulting in a second message, and forwarding the second message to athird device in the network.
 2. The method of claim 1 wherein saidreceiving a first message comprises receiving a first message includingat least one of routing information, a description of the route, andidentification information of the first device in the form of aswitching Type-Length-Value (TLV).
 3. The method of claim 2 wherein saidappending information comprises appending information including at leastone of routing information, a description of the route andidentification information of the second device.
 4. The method of claim1 wherein the first device, the second device and the third device arepart of a Multi-Segment Pseudowire (MS-PW).
 5. The method of claim 1wherein said receiving a first message comprises receiving a firstmessage comprising one of a status message and a setup message.
 6. Themethod of claim 1 wherein said receiving a first message comprisesreceiving a first message including information relating to otherdevices upstream of said second device.
 7. A method of determining apath between network devices comprising: receiving, at an intermediatenetwork device located between a source network device and a destinationnetwork device, a first message including upstream path informationbetween the intermediate device and the source device; receiving at theintermediate network device a second message including downstream pathinformation between the intermediate device and the destination device;and combining, at the intermediate network device, the downstream pathinformation and the upstream path information to determine a completepath from the source network device to the destination network devicewhich includes the intermediate device.
 8. The method of claim 7 whereinsaid receiving a first message includes receiving a first messageincluding at least one of routing information, a description of theroute and identification information of the source device in the form ofa switching TLV.
 9. The method of claim 8 wherein said receiving asecond message includes receiving a second message including at leastone of routing information, a description of the route andidentification information of the destination device in the form of aswitching TLV.
 10. The method of claim 7 wherein the source device, theintermediate device and the destination device are part of aMulti-Segment Pseudowire (MS-PW).
 11. The method of claim 8 wherein saidreceiving a first message includes receiving a first message includinginformation relating to other upstream devices between said sourcedevice and said intermediate network device.
 12. The method of claim 9wherein said receiving a second message includes receiving a secondmessage including information relating to other downstream devicesbetween said destination device and said intermediate network device.13. A network system comprising a first network device, in communicationwith a second network device in communication with a third networkdevice wherein each of the first, second and third network devicescomprises: a memory; a processor; a communications interface; aninterconnection mechanism coupling the memory, the processor and thecommunications interface; and wherein the memory is encoded with anapplication that when performed on the processor, provides a process forprocessing information, the process causing the network devices toperform the operations of: receiving, at a second device in the network,a first message having information relating to a first device in thenetwork; appending, by the second device, information relating to thesecond device to the first message, resulting in a second message, andforwarding the second message to a third device in the network.
 14. Thenetwork system of claim 13 wherein the first message includes at leastone of routing information, a description of the route, andidentification information of the first device in the form of aswitching TLV.
 15. The network system of claim 14 wherein theinformation relating to a second device includes at least one of routinginformation, a description of the route and identification informationof the second device in the form of a switching TLV.
 16. The networksystem of claim 13 wherein the first device, the second device and thethird device are part of a Multi-Segment Pseudowire (MS-PW).
 17. Thenetwork system of claim 14 wherein the first message comprises one of astatus message and a setup message.
 18. The network system of claim 14wherein said first message includes information relating to otherdevices upstream of said second device.
 19. A network device comprising:a memory; a processor; a communications interface; an interconnectionmechanism coupling the memory, the processor and the communicationsinterface; and wherein the memory is encoded with an application fordetermining a path between network devices that when performed on theprocessor, provides a process for processing information, the processcausing the network device to perform the operations of: receiving, anintermediate network device located between a source network device anda destination network device, a first message including upstream pathinformation between the intermediate device and the source device;receiving at the intermediate network device a second message includingdownstream path information between the intermediate device and thedestination device; and combining, at the intermediate network device,the downstream path information and the upstream path information todetermine a complete path from the source network device to thedestination network device which includes the intermediate networkdevice.
 20. The network device of claim 19 wherein the first messageincludes at least one of routing information, a description of the routeand identification information of the source device in the form of aswitching TLV.
 21. The network device of claim 19 wherein the secondmessage includes at least one of routing information, a description ofthe route and identification information of the destination device inthe form of a switching TLV.
 22. The network device of claim 19 whereinthe source device, the intermediate device and the destination deviceare part of a Multi-Segment Pseudowire (MS-PW).
 23. The network deviceof claim 20 wherein said first message includes information relating toother upstream devices between said source device and said intermediatenetwork device.
 24. The network device of claim 21 wherein said secondmessage includes information relating to other downstream devicesbetween said destination device and said intermediate network device.